1. Field of the Invention
The present invention relates to fuel cells and membrane electrode assemblies. More specifically, the present invention relates to a fuel cell and a membrane electrode assembly where a new Pt catalyst that is supported on a carbon carrier is an oxygen electrode catalyst.
2. Description of Related Art
Most part of electric energy has been supplied by thermal power generation, water power generation, and nuclear electric power generation. However, the thermal power generation burns fossil fuel such as oil and coal and it causes not only extensive environmental pollution but also depletion of energy source such as oil. The water power generation requires large-scale dam construction and it causes destruction of nature and has a problem of a limited proper construction site. Further, the nuclear electric power generation has problems that radioactive contamination in the event of accident is fatal and decommissioning of nuclear reactor facility is difficult, and the nuclear reactor construction is decreasing on a global basis.
As a power generation system which does not require large-scale facilities nor causes environmental pollution, wind power generation and solar photovoltaic power generation come into use around the world. Our country also puts the wind power generation and the solar photovoltaic power generation to practical use in some places. However, the wind power generation cannot generate power with no wind and the solar photovoltaic power generation cannot generate power with no sunlight. The two systems are dependent on natural phenomena and thus incapable of stable power supply. Further, the wind power generation has a problem that the frequency of generated power varies by the intensity of wind, causing breakdown of electrical equipment.
Recently, a power plant that draws electric energy out of hydrogen energy, such as hydrogen fuel cells, has been under active development. The hydrogen is obtained by splitting water and exists inexhaustibly on the earth. In addition, the hydrogen has a large chemical energy amount per unit mass, and it does not generate hazardous substance and global warming gas when used as an energy source.
A fuel cell which uses methanol instead of hydrogen has also been studied actively. A direct methanol fuel cell that directly uses methanol, which is a liquid fuel, is easy to use and costs less. Thus, the direct methanol fuel cell is expected to be used as a relatively small output power source for household or industrial use. A theoretical output voltage of a methanol/oxygen fuel cell is 1.2 V (25° C.), which is almost the same as that of the hydrogen fuel cell. Thus, they have the same characteristics in principle.
A polymer electrolyte fuel cell and a direct methanol fuel cell oxidize hydrogen or methanol at the anode and reduce oxygen at the cathode, thereby drawing electric energy. Since oxidation-reduction reaction hardly occurs at room temperature, a catalyst is used in the fuel cells. Initial fuel cells use platinum (Pt) as a catalyst, depositing it on a carbon support. The Pt has catalytic activity for oxidation of hydrogen and methanol. A conventional approach for minimizing Pt catalyst particles to increase a reactive surface area is to control the deposition atmosphere of the Pt catalyst by adjusting external factors in the deposition process. For example, Japanese Unexamined Patent Application Publication No. 56-155645 introduces a technique that, when reducing Pt ion by adding alcohol and depositing it on a carbon support, adds polyvinyl alcohol into a reaction solvent. The polyvinyl alcohol serves as a protective colloid, which absorbs weakly onto the surface of the Pt catalyst particles, thereby forming fine Pt catalyst particles. However, since the protective colloid absorbs onto the surface of the Pt catalyst in this technique, it is necessary to remove the protective colloid from the Pt catalyst surface after catalyst synthesis in order to enhance the catalytic activity. Japanese Unexamined Patent Application Publication No. 56-155645 describes a technique that performs heat treatment at 400° C. in steam flow after catalyst synthesis so as to remove the protective colloid. This processing technique, however, cannot completely remove the protective colloid from the Pt catalyst surface; further, the heat treatment at 400° C. causes sintering of the Pt catalyst particles to increase the catalyst particle size, thereby decreasing the catalytic activity.
If Pt catalyst is synthesized by impregnation, electroless plating (metal plating), or alcohol reduction, the particle diameter is about 2 to 10 nm; thus, the particle diameter distribution is wide and a large number of catalyst particles with a particle diameter of 5 nm or larger exist. Generally, the catalyst activity increases as a surface area per unit weight, which is a specific surface area, increases. If the particle diameter of Pt catalyst is large, the specific surface area is small and catalytic activity is low. Thus, in order to enhance the activity of the Pt catalyst, it is very important to reduce the particle diameter of Pt catalyst to smaller than 5 nm and increase the catalyst specific surface area. In this case, a technique that adds protective colloid to reduce a Pt catalyst particle diameter cannot be used for the above reasons.
Generally, synthesizing Pt catalyst by using a carbon support with a large specific surface area causes the particle diameter of the Pt catalyst to decrease, thus producing Pt catalyst with a large specific surface area, as described in M. Uchida et al., J. Electrochem. Soc. 143, 2245 (1996), for example. This is the same for PtRu catalyst, as described in M. Uchida et al., J. Electrochem. Soc. 142, 2572 (1995), for example. However, the carbon support with a large specific surface area is porous, having an extremely large number of fine pores. Catalyst particles deposited in the fine pores are less involved in oxidation reaction of methanol or hydrogen gas. Therefore, use of a porous carbon support is at odds with the objective that increases the utilization efficiency of expensive PtRu catalyst so as to enhance battery cell characteristics with the smallest amount of catalyst. Thus, though the use of a porous carbon support with a large specific surface area allows synthesizing Pt or PtRu catalyst with a particle diameter of smaller than 5 nm, it has an adverse effect in terms of improvement in utilization efficiency of catalyst. As described above, a trade-off exists between activity improvement and catalyst utilization efficiency improvement by a decrease in catalyst particle diameter due to use of a porous carbon support with a large specific surface area.
On the other hand, use of a non-porous carbon support such as acetylene black and multiwalled carbon nanotube significantly increases catalyst utilization efficiency because all Pt or PtRu catalyst particles are deposited on the support surface. However, since the non-porous carbon support has no pore, its specific surface are is as small as 20 to 140 m2/g. For example, the specific surface area of non-porous multiwalled carbon nanotube is about 20 to 35 m2/g and the specific surface area of non-porous acetylene black is about 60 to 140 m2/g. The specific surface area of these non-porous carbon supports is smaller by one to two digits than that of Ketjenblack 600 JD that is a porous carbon support, which is 1270 m2/g. Synthesizing Pt or PtRu catalyst by using a carbon support with a small specific surface area causes a particle diameter to increase as described in M. Uchida et al., J. Electrochem. Soc. 143, 2245 (1996) and M. Uchida et al., J. Electrochem. Soc. 142, 2572 (1995) mentioned above. Thus, use of a non-porous carbon support with a small specific surface area results in a larger particle diameter of Pt or PtRu-catalyst. The specific surface area of the Pt or PtRu catalyst thereby decreases to reduce catalyst activity. Therefore, a trade-off that is similar to the case of using a porous carbon support with a large specific surface area also exists in the case of using a non-porous carbon support with a small specific surface area.